The present invention generally relates to manufacture of devices. More particularly, the present invention provides a method and device using photonic crystals or the like in combination with optical devices composed of a gallium-containing nitride crystal, but there can be others. More specifically, embodiments of the invention include techniques for fabricating a light emitting diode device using bulk gallium nitride containing materials and the like. Merely by way of example, the invention can be applied to applications such as optoelectronic devices, and the like. In other embodiments, the present invention provides a method of manufacture using a high quality epitaxial gallium containing crystal with a release layer, but it would be recognized that other crystals and materials can also be processed. Such crystals and materials include, but are not limited to, GaN, AN, InN, InGaN, AlGaN, and AlInGaN, and others for manufacture of bulk or patterned substrates.
Progress has been made during the past decade and a half in the performance of gallium nitride (GaN) based light emitting diodes (LEDs). Devices with a luminous efficiency higher than 100 lumens per watt have been demonstrated in the laboratory, and commercial devices have an efficiency that is already considerably superior to that of incandescent lamps and competitive with that of fluorescent lamps. Further improvements in efficiency are desired in order to reduce operating costs, reduce electricity consumption, and decrease emissions of carbon dioxide and other greenhouse gases produced in generating the energy used for lighting applications.
The efficiency of LEDs is limited in part by the internal quantum efficiency and by the light extraction efficiency. The internal quantum efficiency can be improved by the use of bulk gallium nitride substrates, with low concentrations of threading dislocations, and by the use of nonpolar and semipolar crystallographic orientations, which reduce or eliminate the presence of deleterious polarization electric fields within the device.
The light extraction efficiency can be improved by the formation of a photonic crystal structure on the surface. For example, Wierer et al. [Appl. Phys. Lett. 84, 3885 (2004)] demonstrated an InGaN/GaN LED with a triangular lattice photonic crystal dry-etched into the top layer. The photonic crystal pattern modified the far-field emission pattern and increased the light extraction efficiency. However, this structure involved top-surface emission, with the substrate still present. As noted by David et al. [Appl. Phys. Lett. 88, 061124 (2006)], in the absence of special efforts to confine the light to the near-surface region, light extraction remains relatively inefficient due to the poor overlap of low-order light propagation modes with the photonic crystal. In addition, the relatively poor electrical conductivity of the p-type layer makes it difficult to avoid ohmic losses in high power LEDs. One way to overcome these problems is by thinning or removal of the substrate. For example, David et al. [Appl. Phys. Lett. 88, 133514 (2006)] fabricated an LED with a photonic crystal structure on the backside of the n-type layer, exposed by flip-chip bonding of the device to a submount and performing laser liftoff of the sapphire substrate. However, this latter approach suffers from several limitations. Most importantly, the use of a sapphire or other non-GaN substrate, while providing a natural means for removal of the substrate, does not provide a means for achievement of very low dislocation densities in the active layer, which may negatively impact the internal quantum efficiency. In addition, the electron-beam lithography technique employed by David et al. does not lend itself to cost-effective manufacturing, and the reflectivity of the p-type electrical contact was undesirably low.
U.S. Pat. No. 7,053,413, hereby incorporated by reference in its entirety, teaches fabrication of a homoepitaxial LED on a bulk GaN substrate with a dislocation density below 104 cm−2, followed by removal of a portion of the substrate. However, the only means taught for removal of the portion of the substrate are lapping, polishing, chemical etching, plasma etching, and ion beam etching. These methods do not provide a natural endpoint, and it is therefore extremely difficult to remove all but a few- or sub-micron-thick layer of uniform thickness, and are slow and expensive to perform.
What is needed is a manufacturable means for fabricating a thin, photonic-crystal LED with improved light extraction capability and, simultaneously, an ultralow defect density and high crystallographic quality device structure with a high internal quantum efficiency.